TO procname :input1 :input2 ...				(special form)

	command.  Prepares Logo to accept a procedure definition.  The
	procedure will be named "procname" and there must not already
	be a procedure by that name.  The inputs will be called "input1"
	etc.  Any number of inputs are allowed, including none.  Names
	of procedures and inputs are case-insensitive.

	Unlike every other Logo procedure, TO takes as its inputs the
	actual words typed in the instruction line, as if they were
	all quoted, rather than the results of evaluating expressions
	to provide the inputs.  (That's what "special form" means.)

	This version of Logo allows variable numbers of inputs to a
	procedure.  After the procedure name come four kinds of
	things, *in this order*:

	    1.   0 or more REQUIRED inputs    :FOO :FROBOZZ
	    2.   0 or more OPTIONAL inputs    [:BAZ 87] [:THINGO 5+9]
	    3.   0 or 1 REST input            [:GARPLY]
	    4.   0 or 1 DEFAULT number        5

	Every procedure has a MINIMUM, DEFAULT, and MAXIMUM
	number of inputs.  (The latter can be infinite.)

	The MINIMUM number of inputs is the number of required inputs,
	which must come first.  A required input is indicated by the

			:inputname

	notation.

	After all the required inputs can be zero or more optional inputs,
	each of which is represented by the following notation:

			[:inputname default.value.expression]

	When the procedure is invoked, if actual inputs are not supplied
	for these optional inputs, the default value expressions are
	evaluated to set values for the corresponding input names.  The
	inputs are processed from left to right, so a default value
	expression can be based on earlier inputs.  Example:

			to proc :inlist [:startvalue first :inlist]

	If the procedure is invoked by saying

			proc [a b c]

	then the variable INLIST will have the value [A B C] and the
	variable STARTVALUE will have the value A.  If the procedure
	is invoked by saying

			(proc [a b c] "x)

	then INLIST will have the value [A B C] and STARTVALUE will
	have the value X.

	After all the required and optional input can come a single "rest"
	input, represented by the following notation:

			[:inputname]

	This is a rest input rather than an optional input because there
	is no default value expression.  There can be at most one rest
	input.  When the procedure is invoked, the value of this inputname
	will be a list containing all of the actual inputs provided that
	were not used for required or optional inputs.  Example:

			to proc :in1 [:in2 "foo] [:in3 "baz] [:in4]

	If this procedure is invoked by saying

			proc "x

	then IN1 has the value X, IN2 has the value FOO, IN3 has the value
	BAZ, and IN4 has the value [] (the empty list).  If it's invoked
	by saying

			(proc "a "b "c "d "e)

	then IN1 has the value A, IN2 has the value B, IN3 has the value C,
	and IN4 has the value [D E].

	The MAXIMUM number of inputs for a procedure is infinite if a
	rest input is given; otherwise, it is the number of required
	inputs plus the number of optional inputs.

	The DEFAULT number of inputs for a procedure, which is the number
	of inputs that it will accept if its invocation is not enclosed
	in parentheses, is ordinarily equal to the minimum number.  If
	you want a different default number you can indicate that by
	putting the desired default number as the last thing on the
	TO line.  example:

			to proc :in1 [:in2 "foo] [:in3] 3

	This procedure has a minimum of one input, a default of three
	inputs, and an infinite maximum.

	Logo responds to the TO command by entering procedure definition
	mode.  The prompt character changes from "?" to ">" and whatever
	instructions you type become part of the definition until you
	type a line containing only the word END.